The present invention relates to the field of actuators designed to displace an object from a first position as far as at least one second position, and that have a shape-memory element.
Shape-memory-actuator (SMA) elements have been known for some time and used in wide range of fields. They use shape-memory metal alloys capable of undergoing deformation when a pre-set transition temperature is exceeded. In general, heating can be obtained in so far as the SMA element detects directly a variable temperature, or else by supplying an electric current through the SMA element so as to heat it by the Joule effect.
One of the applications of shape-memory elements is in actuators.
The U.S. Pat. No. 6,006,522 describes an actuator that includes a strip made of a shape-memory material, arranged around a fixed element and connected to an object, in such a way that, when the shape-memory element is activated by means of a heating element, the martensitic/austenitic transformation causes displacement of the aforesaid object. Said transformation leads to a slight shortening of the strip. The latter, however, is not free to slide in so far as it is in frictional contact with the aforesaid heating element so that the turns of the strip are subjected to a radial movement towards the outside and to a circumferential movement.
Consequently, in this type of known actuator the effective displacement of the object is not caused by the variation in the length of the shape-memory strip but by the increase in the radius of its turns. Consequently, the control performed by this actuator will prove far from precise and difficult to calibrate as a function of the temperature reached by the shape-memory element itself. In addition, for proper operation of this type of actuator, the space inside the casing that encloses it will have to be such as to enable the necessary expansion.
The U.S. Pat. No. 5,396,769 shows an actuator that has an elongated SMA element wound in a helix on an element that is free to rotate. In this case, the displacement governed by the actuator depends directly upon the variation in length of the SMA element. However, the variation in length of the SMA element is countered by the friction that acts between the surfaces of the turns mutually in contact.
In addition to preventing shortening of the SMA element, friction can also generate a radial and circumferential movement outwards (as in the previous case), which albeit leading to the effective displacement of the object moved by the actuator, leads to an operation that is far from precise and in this case undesirable.
The above drawback is intrinsic in the windings of shape-memory elements in so far as the presence of friction, both between the SMA element and the surface on which it is wound and between the various portions of surface of the SMA element, creates a field of forces that tend to slacken and even modify the winding of the SMA element.